What is the principle of the recently popular thermosensitive mobile phone case?

Heat-sensitive mobile phone case is also called heat-sensitive mobile phone case. Simply put, it is similar to a color-changing cup. When the temperature on the hand is conducted to the corresponding place of the mobile phone case, it will change color. When black changes color, it will be fluorescent orange, and when red changes color, fluorescent orange will gradually turn yellow. It is a very fashionable mobile phone case! Only ornamental value!